So i had this same problem on my site.
It appears you have a child div set to 100% width, or something along that line.
So when you shrink the window, the white background shrinks to 100% of the current window size, but the min-width keeps the text from collapsing and causes the grey background to appear.
I often browse with my window at a smaller size, so its noticeable to me. Not that high priority, it just looks ugly.